The invention relates to fusing in electrostatography, and more particularly to an improved fusing station having an externally heated fuser roller for fixing a toner image to a receiver member.
In electrostatographic imaging and recording processes such as electrophotographic reproduction, an electrostatic latent image is formed on a primary image-forming member such as a photoconductive surface and is developed with a thermoplastic toner powder to form a toner image. The toner image is thereafter transferred to a receiver member, e.g., a sheet of paper or plastic, and the toner image is subsequently fused or fixed to the receiver member in a fusing station using heat and/or pressure. The fusing station includes a fuser member which can be a roller, belt, or any surface having a suitable shape for fixing thermoplastic toner powder to the receiver member. The fusing step using a roller fuser member commonly includes passing the toned receiver member between a pair of engaged rollers that produce an area of pressure contact known as a fusing nip. In order to form the fusing nip, at least one of the rollers typically includes a compliant or conformable layer. Heat is transferred from at least one of the rollers to the toner in the fusing nip, causing the toner to partially melt and attach to the receiver member. In the case where the fuser member is a deformable heated roller, a resilient elastomeric layer is typically bonded to the core of the roller, with the roller having a smooth outer surface. Where the fuser member is in the form of a belt, e.g., a flexible endless belt that passes around the heated roller, it typically has a smooth outer surface which may also be hardened.
Simplex fusing stations attach toner to only one side of the receiver member at a time. In this type of station, the engaged roller that contacts the unfused toner is commonly known as the fuser roller and is a-heated roller. The roller that contacts the other side of the receiver member is known as the pressure roller and is usually unheated. Either or both rollers can have a compliant layer on or near the surface. It is common for one of these rollers to be driven rotatable by an external source while the other roller is rotated frictionally by the nip engagement.
It is known that a resilient fuser roller, when used in conjunction with a harder or relatively non-deformable pressure roller, e.g., in a Digimaster 9110 machine made by Heidelberg Digital LLC, provides easy release of a receiver member from the fuser roller, because the distorted shape of the compliant surface in the nip tends to bend the receiver member towards the relatively non-deformable unheated pressure roller and away from the much more deformable fuser roller. A pressure roller may advantageously be provided with a polymeric outermost coating, such as the pressure roller disclosed in the Chen et al. patent application (U.S. patent application Ser. No. 09/957,992, filed Sep. 21, 2001).
The most common type of fuser roller is internally heated, i.e., a source of heat is provided within the roller for fusing. Such a fuser roller generally has a hollow core, inside of which is located a source of heat, usually a lamp. Surrounding the core can be an elastomeric layer through which heat is conducted from the core to the surface, and the elastomeric layer typically contains fillers for enhanced thermal conductivity.
Less common is an externally heated fuser roller, such as for example used in an Image Source 120 copier marketed by Eastman Kodak Company, which fuser roller is typically heated by surface contact with one or more heating rollers. Externally heated fuser rollers are disclosed by the O""Leary patent (U.S. Pat. No. 5,450,183), the Derimiggio et al. patent (U.S. Pat. No. 4,984,027), the Stack et al. patent application (U.S. patent application Ser. No. 09/680,134, filed Oct. 4, 2000), and the Chen et al. patent application (U.S. patent application Ser. No. 09/680,138, filed Oct. 4, 2000).
A conventional toner fuser roller includes a rigid cylindrical core member, typically metallic such as aluminum, coated with one or more synthetic layers usually formulated with polymeric materials made from elastomers. A resilient base cushion layer, which may contain filler particles to improve mechanical strength and/or thermal conductivity, is typically formed on the surface of the core, which may advantageously be coated with a primer to improve adhesion of the resilient layer. Roller cushion layers are commonly made of silicone rubbers or silicone polymers such as, for example, polydimethylsiloxane (PDMS) polymers disclosed by the Chen et al. patents (U.S. Pat. Nos. 5,960,145 or 6,020,038).
Some roller fusers rely on film splitting of low viscosity oil to enable release of the toner and (hence) receiver member from the fuser roller. The oil is typically applied to the surface of the fuser from a donor roller coated with the oil provided from a supply sump. A donor roller is disclosed in the Chen et al. patent (U.S. Pat. No. 6,190,771) and in the Chen et al. patent application (U.S. patent application Ser. No. 09/960,661, filed Sep. 21, 2001).
Release oils (commonly referred to as fuser oils) are composed of, for example, polydimethylsiloxanes. When applied to the fuser roller surface to prevent the toner from adhering to the roller, fuser oils may, upon repeated use, interact with PDMS material included in the resilient layer(s) in the fuser roller, which in time can cause swelling, softening, and degradation of the roller. To prevent these deleterious effects caused by release oil, a thin barrier layer made of, for example, a cured fluoroelastomer and/or a silicone elastomer, is typically formed on the resilient cushion layer, as disclosed in the Davis et al. patent (U.S. Pat. No. 6,225,409).
To rival the photographic quality produced using silver halide technology, it is desirable that electrostatographic multicolor toner images have high gloss. To this end, it is desirable to provide a very smooth fusing member contacting the toner particles in the fusing station. A fuser roller having improved gloss characteristics is disclosed in the Chen et al. patent application (U.S. patent application Ser. No. 09/608,290, filed Jun. 30, 2000). A fluorocarbon thermoplastic random copolymer useful for making a gloss control coating on a fuser roller is disclosed in the Chen et al. patent application (U.S. patent application Ser. No. 09/609,561, filed Jun. 30, 2000).
In the fusing of the toner image to the receiver member, the area of contact of a conformable fuser roller with the toner-bearing surface of a receiver member sheet as it passes through the fusing nip is determined by the amount pressure exerted by the pressure roller and by the characteristics of the resilient cushion layer. The extent of the contact area helps establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser roller.
As previously mentioned, PDMS cushion layers may include inorganic particulate fillers, such as for example made of metals, metal oxides, metal hydroxides, metal salts, and mixtures thereof. The Fitzgerald patent (U.S. Pat. No. 5,292,606) describes fuser roller base cushion layers that contain fillers of particulate zinc oxide and zinc oxide-aluminum oxide mixtures. Similarly, the Fitzgerald patent (U.S. Pat. No. 5,336,539) describes a fuser roller cushion layer containing dispersed nickel oxide particles. Also, the fuser roller described in the Fitzgerald et al. patent (U.S. Pat. No. 5,480,724) includes a base cushion layer containing 20 to 40 volume percent of dispersed tin oxide particles.
Filler particles may also be included in a barrier layer. For example, the Chen et al. patent (U.S. Pat. No. 5,464,698) discloses a toner fuser member having a silicone rubber cushion layer and an overlying barrier layer of a cured fluorocarbon polymer in which is dispersed a filler comprising a particulate mixture that includes tin oxide.
The Chen et al. patents (U.S. Pat. Nos. 5,960,145 or 6,020,038) disclose an improved fuser roller including three concentric layers each containing a particulate filler, i.e., a base cushion layer made from a condensation-cured PDMS, a barrier layer covering the base cushion made of a cured fluorocarbon polymer, and an outer surface layer made of an addition-cured PDMS, with particulate fillers in the layers including one or more of aluminum oxide, iron oxide, calcium oxide, magnesium oxide, tin oxide, and zinc oxide. The barrier layer may include a Viton(trademark) elastomer (sold by DuPont) or a Fluorel(trademark) elastomer (sold by Minnesota Mining and Manufacturing).
Prior art internally heated conventional fuser rollers typically have one or more synthetic polymeric layers including a deformable layer such as a base cushion layer surrounding a hollow metallic core member, with a source of heat such as a lamp provided within the hollow core member. Such fuser rollers rely on thermal conductivity through the synthetic layers for conduction of heat from the source of heat to the surface of the roller so as to provide heat for fusing toner particles to receiver members. The thermal conductivity, attainable by the use of one or more suitable particulate fillers, is determined by the filler concentration. The thermal conductivity of most polymers is very low and the thermal conductivity generally increases as the filler concentration is increased. However, if the filler concentration is too high, the mechanical properties of a polymer are usually compromised. For example, the stiffness of the synthetic layers may be increased by too much filler so that there is insufficient deformability to create a wide enough nip for proper fusing. Moreover, too much filler will cause the synthetic layers to have a propensity to delaminate or crack or otherwise cause failure of the roller. Because the mechanical requirements of such an internally heated fuser roller require that the filler concentrations be moderate, the ability of the roller to transport heat is thereby limited. In fact, the concentration of filler in prior art internally heated deformable fuser rollers has reached a practical maximum. As a result, the number of copies that can be fused per minute is limited, and this in turn can be the limiting factor in determining the maximum throughput rate achievable in an electrostatographic printer. There is a need, therefore, to provide an improved fusing station for increasing the increasing the number of prints that can be fused per minute, thereby providing opportunity for higher machine productivity.
An auxiliary internal source of heat may optionally be used.with an externally heated fuser roller, e.g., as disclosed in the Stack et al. patent application (U.S. patent application Ser. No. 09/680,134, filed Oct. 4, 2000) and in the Chen et al. patent application (U.S. patent application Ser. No. 09/680,138, filed Oct. 4, 2000). Such an internal source of heat is known to be useful when the fusing station is quiescent and/or during startup when relatively cold toned receiver members first arrive at the fusing station for fusing therein. It will be evident from the preceding paragraph above that in order for such an auxiliary internal source of heat to be effective (when intermittently needed) the fuser roller must have a sufficiently large thermal conductivity. However, this requirement conflicts with a need to keep heat at the surface of an externally heated fuser roller, i.e., so as not to unnecessarily conduct heat into the interior which would compromise the fusing efficiency of the roller.
Thus there remains a need to provide an improved efficiency fusing station so that the throughput rate can be increased over that of prior art. In particular, there remains a need for an externally heated fuser roller having an optimized rate of thermal conduction from the surface to the interior and vice versa, such that the fuser roller can be optionally intermittently and efficiently heated by an auxiliary internal source of heat.
Accordingly, this invention is directed to a fusing station for fusing toner images to receiver members, the fusing station including a deformable fuser member in pressure engagement with a relatively harder pressure roller, the fuser member incorporating a heat storage layer, the fuser member heated by an external source of heat. A deformable fuser member in the form of a roller includes an annular base cushion layer around a rigid cylindrical core member, with an annular heat storage layer around the base cushion layer, and a thin annular gloss control layer around the heat storage layer. The base cushion layer is less thermally conductive than the heat storage layer, and a thermal conductivity of the heat storage layer divided by a thermal conductivity of the base cushion layer is a preselected ratio having a value preferably in a range of approximately between 1.5-7. By comparison with a prior art fuser roller having a nominal fusing temperature and operated at a baseline throughput rate with a given external heating load, the subject fuser roller at the same nominal fusing temperature has an improved fusing efficiency, the fusing station thereby having a higher throughput rate of fused receiver members for the same external heating load. Alternatively, the improved efficiency permits the external source of heat to use a smaller heating load when fusing at the same nominal fusing temperature and the same baseline throughput rate.
The base cushion layer of the fuser roller preferably has a thermal conductivity in a range of approximately between 0.1 BTU/hr/ft/xc2x0 F.-0.2 BTU/hr/ft/xc2x0 F., the heat storage layer preferably has a thermal conductivity in a range of approximately between 0.3 BTU/hr/ft/xc2x0 F.-0.7 BTU/hr/ft/xc2x0 F., and the gloss control layer preferably has a thermal conductivity greater than about 0.07 BTU/hr/ft/xc2x0 F. A ratio of thermal conductivity divided by thickness for the base cushion layer has a preselected value preferably in a range of approximately between 4.8 BTU/hr/ft2/xc2x0 F.-13.3 BTU/hr/ft2/xc2x0 F., a ratio of thermal conductivity divided by thickness for the heat storage layer has a preselected value preferably in a range of approximately between 300 BTU/hr/ft2/xc2x0 F.-1400 BTU/hr/ft2/xc2x0 F., and a ratio of thermal conductivity divided by thickness for the gloss control layer has a preselected value preferably in a range of approximately between 380 BTU/hr/ft2/xc2x0 F.-880 BTU/hr/ft2/xc2x0 F.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.